Make it impossible for websites to style forms in Debian Blizzardhawk

["Jordi Gutiérrez Hermoso" <jordigh@gmail.com>]

Blizzardhawk, Fireweasel, Icewolf, whatever...

Anyways, the issue is that I modified my GTK+ theme because I like
dark themes, and on some websites, all the form elements (buttons,
text boxes, radio buttons, checkmarks), look lovely:

     http://sums.math.mcgill.ca/~jordi/piccies/exhibit-a.png

Yeah, they clash with the rest of the website design a little, but
whatever. I like it that way.

The problem is that in this modern web of ours, it's a favourite fad
to use all these fancy colours all over the place and style form
elements in ways that may look broken and unreadable because they
clash with our native theme (and since nobody uses light-on-dark
themes, it's ok to assume that we can use a light-coloured background
or dark-coloured text when we style our forms, right?):

     http://sums.math.mcgill.ca/~jordi/piccies/exhibit-b.png

Basically, I'd like to make it impossible for websites to use colours
for forms at all, *but only forms*. Of course I can tell Mozilla
Webarachnid to not use any colours from the webpage at all, but that's
a bit extreme and monochromatic:

     http://sums.math.mcgill.ca/~jordi/piccies/exhibit-c.png

I know that I can probably accomplish what I want by modifying some
CSS, but there are is so many CSS in various locations that I don't
know how to actually do it... perhaps with a Stylish theme, or with
userChrome.css or modify /usr/share/xulrunner-1.9/res/forms.css, or
what? What I want is for all forms elements to use my Gnome theme,
regardless of what the website author thinks the form element should
look like.

For added bonus, I'd like to accomplish this without the need of root
privileges.

Is this possible?

Thanks in advance,
- Jordi G. H.